There will be people who agree with the following because it matches their experience, and there will be people who disagree with every word because... (these are the same people who will swear blind that it never rains in the Wales because the one day they visited it was sunny).
Having owned 8 Omegas and maintained them myself, all face lift, both saloon and estate, manual and auto, 2.0/2.2/2.6/3.2 and near as damnit 250,000 miles combined, not to mention countless members cars that I have worked on, here's my experience based list of common issues that will affect any Omega, especially between 80-120k miles and beyond depending on maintenance approach. Sensors should only be genuine, from a VX dealer that you genuinely have to walk in to. Everything else as available.
In no particular order:
1. V6 crank sensor (non start, no codes).
2. 4 pot cam sensor (especially with gearbox codes).
3. Breathers (see #4, long oil changes/crappy oil will cause them to sludge up and blow the camcover seals)
4. Cam cover seals(see #3 also go brittle over time leaks will destroy the coil packs).
5. Coil packs (see #4 and also destroyed by water leaks from the scuttle/bulkhead foam).
6. Fuel pump (especially if lpg and tank left low).
7. Heater Bypass Valve (can last anything from 6 months to 4 years and can cause catastrophic coolant loss if unnoticed).
8. Coolant exansion tank (microscopic cracks around the filler neck lead to mysterious coolant loss).
9. Wishbone bushes (the Omega is pretty rough on bushes if driven hard and whilst seemingly simple, quite fussy about geometry settings).
10. Rear discs (available as vented: 2.6/3.2 or solid: everything else, but they have different calipers and not unusual to find solid discs with vented calipers this will allow the pistons to leak as the pads wear resulting in brake failure)
11. Rust. Everywhere between the number plates, but especially front chassis rails, bulkhead, sills, rear arches, saloon rear shock top mounts and rear crossmember.
As with any car, they will effortlessly cover mileage but they are better used than sat around. Higher mileage, low owner, meticulously maintained cars are a better daily driver buy than a low mileage garage queen.
As for the forum, most items are covered by the FAQ and Maintenance Guide sections, and if you go back to when the Omega Help section was 15 pages a month, you'll see pretty much every issue people have encountered.
Drive an Omega long enough and you'll encounter these repeatedly. Equally, buy cheap Omegas and you'll encounter everything. Every time. And that quickly gets old...